Colorado regulators said on Thursday that the disposal of oil and gas wastewater at a well in Weld County likely caused a series of small earthquakes this year, in another sign that a U.S. drilling boom is contributing to higher seismic activity.

The issue of wastewater disposal disturbing underground fault lines has become a national issue in the United States where drilling and wastewater disposal have increased sharply in recent years.

The Colorado Oil and Gas Conservation Commission (COGCC) halted activity at a wastewater disposal site owned by NGL Water Solutions DJ LLC on June 23, after a 3.2 magnitude quake on May 31 was followed by other smaller tremblers in the area. COGCC is investigating whether the company exceeded permitted injection volumes.

Since the shutdown, the seismic activity continued but occurred at “lower levels” COGCC said in a statement. “Actions at the location are potentially related to low-level seismic activity nearby,” it said.

Limited operations have been allowed to continue at the disposal site after NGL Water Solutions sealed off what the COGCC described as a “preferential pathway” for the wastewater to flow from the bottom of the well into basement rock beneath.

COGCC will continue to monitor the Weld County well and will halt work if another seismic event above 2.5 occurs.

This is likely the first time that seismic activity has been linked to wastewater disposal, a COGCC spokesman said last month.

Hydraulic fracturing, or fracking, is an oil and gas production technique that involves pumping millions of gallons of water underground to release oil and gas. Much of that water comes back to the surface after drilling and is disposed of in large underground wells.

There were about 145,000 of these wells in the United States in 2012 and 309 in Colorado, according to the Colorado Geological Survey.

Both fracking and wastewater disposal have been linked to increased seismic activity in states where energy production is on the rise.

Recent small earthquakes in Ohio were likely triggered by fracking, state regulators said in April, establishing a new link that went beyond just the impacts of disposal wells.

(Reporting By Edward McAllister; Editing by Bernard Orr)